The present invention concerns in particular low-cost infrared (IR) gas detection as disclosed in WO 2005/026705 A1.
The gas detection method and gas detector device as described in this prior art publication is based on a source formed by a wavelength modulated Vertical Cavity Surface Emitting Laser (VCSEL) or Distributed FeedBack (DFB) laser and uses the fact that the modulation of the wavelength is directly connected to a modulation of the laser source output intensity. The intensity of the light having passed the gas volume and being incident to the detector therefore shows a first modulation related to the laser source intensity and a second modulation related to the gas absorption as the wavelength is scanned across the gas absorption line. Accordingly, the known detection method and device provides an initial light signal by a wavelength modulated laser source.
The source provides an initial light signal, which is wavelength modulated with an AC modulation signal at a given initial frequency (f) at the absorption line around the gas to be determined. A light sensor respectively is arranged at the periphery of a detection region intended for receiving at least a gas the concentration of which is to be determined. The light sensor receives a resulting light signal formed by the initial light signal having passed through the detection region. In the following a detection signal is formed which is substantially proportional to the time derivate of the resulting light signal. Further disclosed are first means for generating a first modulation reference signal at the given frequency (f) and second means for generating a second modulation reference signal at twice this frequency (2f). The detection signal is multiplied by the first modulation reference signal and then integrated over time in order to provide a first measuring signal which is a function of the intensity of said initial light signal and substantially independent of the concentration of said gas. The detection signal is further multiplied by said second modulation reference signal and then integrated over time in order to provide a second measuring signal which is a function of the gas absorption and substantially independent of an intensity modulation of the initial light signal at the given initial frequency. The final measuring signal is then received by dividing the second measuring signal by the first measuring signal, thereby providing a signal relative to the concentration or the presence of a given gas. This gas detector method and device have the advantage that only a single sensor unit is needed for one laser source. All necessary information for determining a precise gas concentration value is given by the processing of the generated detection signal which is proportional to the derivate of the light signal received by the sensor unit after having passed through a sample of the defined gas.
The first and second reference modulation signal both are in phase with the intensity variations of the initial light signal. With this known measurement technique the detector signal is time derivated, and the derivated signal is fed into a two-channel lock-in amplifier. The first channel operates on the modulation frequency f, and the output signal is proportional to the slope of the optical power as function of the laser current. The second channel operates of twice the modulation frequency and its output gives a signal, which is proportional to the gas concentration encountered by the laser beam. The ratio of the measuring signal at the frequency 2f to the measuring signal at the frequency f gives the absolute concentration of the gas independent of the laser output as the measuring signal at the frequency f contains information about the laser intensity under the assumption that variations of the laser intensity stem from optical degradations in the light path, such as dust, condensation, speckles. This assumption only holds for two conditions:
1. The laser does not show mode hopping, i.e. sudden changes of wavelength. If such a mode hopping occurs, the wavelength has to be re-adjusted by a change of the DC laser current, which in turn changes the laser output power. With a VCSEL the slope, which is measured by the signal at the frequency f does not necessarily change accordingly. In the case of a DFB laser, the output power is strictly proportional to the DC current which gives the same signal at the frequency f for different output powers.
2. The temperature of the laser is exactly stabilized. For a change of the laser temperature, the wavelength changes, which in turn leads to a re-adjustment of the DC laser current to stay centered on the wavelength of the gas absorption line. Such a change of the current means an intensity change as described in item 1.
With the method described in the prior art patent application, the signal based on a modulation reference signal at the frequency f shows a slope around the center of the gas absorption line, which is proportional to the gas concentration. At high gas concentrations, the accuracy of the measurement is limited by the accuracy of the DC laser current of which the error influences the modulation reference signal at the frequency f. Variations of the current will cause variation of the laser signal, and this effect increases with concentration. This shows, that for some applications this prior art method and device is quite demanding in terms of temperature control of the laser, and depends very much on the thermal mounting of the latter. DFB lasers and VCSEL's differ very much in their thermal budget so that the tracking of the gas absorption line, which is always necessary in term of DC current, has to include a temperature tracking as well.
The co-pending U.S. patent application Ser. No. 11/227,477 describes a first modulation reference signal at twice of said initial frequency is generated by respective means, whereby said first modulation reference signal has a 45° phase angle to said initial light signal. This first modulation reference signal oscillates at an amplitude level between amplitude levels 1 and 0 and is different from the amplitude level of the second modulation reference signal. Finally the detection signal directly received from the resulting light signal is multiplied with the first modulation reference signal. Thus, the first modulation reference signal is not measured on the frequency f, but on the frequency 2f with a slight modification of the 2f modulation reference signal in the amplitude levels and a phase shifting of 45° between the first modulation reference signal and the initial frequency, which is necessary to provide the same phase which is obtained by a derivate over time. Further, the detector signal is no longer derivated but directly fed to the lock-in amplifier for generating a first measuring signal, which is a function of the intensity of the initial light signal. The resulting signal is directly proportional to the light intensity of the laser as seen by the detector without gas absorption (i.e. including any degradations of the light beam between laser and detector). Further it is proposed to combine this first 2f modulation reference signal and its signal treatment with other treatments in order to obtain stable final measuring signals dependent on the special application of gas detection. In a further embodiment, the second modulation reference signal is generated at twice of said initial frequency f, whereby the first and second modulation reference signals have the same phase correlation to the initial light signal; therefore both signals have 45° phase angle to the AC modulation signal for the laser source. Further, the second modulation reference signal oscillates between amplitude levels 1 and −1. For generating the second measuring signal the detection signal directly received from the resulting light signal is multiplied via lock-in amplifier with said second modulation reference signal. The final measuring signal is obtained by the above-mentioned ratio. In this embodiment the final measuring signal is obtained by a first and a second measuring signal based on a 2f modulation reference signal, both obtained with a detection signal directly received from the resulting light signal. In an other embodiment the second modulation reference signal is generated at twice of said initial frequency f, whereby said second modulation reference signal is exactly in phase with the intensity variations of said initial light signal. The detection signal generated by said detection means is substantially proportional to the time derivate of said resulting light signal and the second measuring signal is generated by multiplying said detection signal with said second modulation reference signal. This signal treatment shows the best result, which is independent from the laser temperature and sudden wavelength changes. In this embodiment also the final measuring signal is obtained by a first and a second measuring signal based on a 2f modulation reference signal, but the second measuring signal, which is a function of the absorption is obtained with a derivated detection signal. In a further embodiment, which needs more electronic parts, two reference modulation signals at a frequency f and 2f are used for generating two measuring signals, which are a function of intensity of the initial light signal. This is realised by generating, additionally to the first measuring signal based on the first 2f modulation reference signal, a third measuring signal, which is also a function of intensity of said initial light signal. This third measuring signal is generated from a detection signal by multiplying the detection signal with a third modulation reference signal at the initial frequency f and then integrated over time. Further the second measuring signal is generated from said detection signal, by multiplying said detection signal with a second 2f modulation reference signal at twice of said initial frequency f and then integrated over time. The third and second modulation reference signals are exactly defined in phase with the intensity variations of said initial light signal and the detection signal for both measuring signals are substantially proportional to the time derivate of the resulting light signal. The final measuring signal is obtained by correlating the first and third measuring signal and generating the ratio between the second measuring signal and the correlated signal of the first and second measuring signal.
Generally speaking, in wavelength modulation laser spectrometry, the laser wavelength is modulated at a modulation frequency f. After transmission of the light through the gas sample to be measured, the laser light is incident onto a photo detector. In general, the signal of the photo detector is fed into a phase-sensitive lock-in amplifier and the gas concentration is related to the photo detector signal on twice the modulation frequency (2f detection).
The 2f detection is limited by various noise sources: laser intensity noise (which partly can be compensated for by measuring the laser intensity);
electronic noise stemming from the photo detector and/or the amplifying circuitry;
optical interference-based noise.
The optical noise based on interference can take the form of speckles or of etalon fringes. Speckles are interference patterns created by the diffraction of the coherent laser light at irregularities like dust, dirt etc. Speckles are not the object of this invention.
Etalon fringes are caused by portions of the light, which are back-reflected from optical interfaces within the designed light path (i.e. windows, lenses, mirrors etc.). As the back-reflected parts of the laser light are in coherence with the laser beam, the interaction of back-reflection and propagating laser light can create a standing wave within the cavity of the gas absorption device (i.e. the free-space absorption path containing the gas to be measured).
If the length of the cavity is changed, the amplitude of the standing wave at the location of the photo detector will vary between periodical minima and maxima as a function of the cavity length. The same effect can be obtained if the cavity length is held constant and the wavelength of the laser light is changed. The periodical variation of the light amplitude with cavity length or with laser wavelength is called “etalon fringes”. FIG. 1 shows in diagram (a) etalon fringes of a methane gas detector with zero gas concentration (note that the electronic noise floor is not resolved in this graph) and in diagram (b) the same device as in (a) with methane present. The etalon fringes are about the same size as the gas absorption signal at the center of the absorption peak.
In a real-world gas detection device, the variation of the length of the cavity stems from thermal expansion of its mechanical members. As an example, with a thermal expansion coefficient of steel (10 ppm/° C.), a mechanical length of 10 cm and a temperature of operation from 0° C. to 50° C., the cavity changes its length by 5 micrometers. This variation of length corresponds to 3 to 4 times the laser wavelength and can therefore create massive etalon fringes. The etalon fringes are not directly correlated to the intensity of the laser and can therefore not be compensated by measuring the laser intensity.
The period of the etalon fringes on the ambient temperature or the laser wavelength (drive current) is a function of the length of the geometrical cavity, which creates the etalon fringes: The longer the cavity, the shorter the etalon fringe period. For a gas detection device based on a Vertical Cavity Surface Emitting Laser (VCSEL), the etalon fringe period is on the same order than the gas absorption peak (in wavelength) if the etalon fringe generating cavity has a length of several centimeters, which is the typical length of an absorption path. Very small cavities, i.e. the window of the laser cap, generate etalon fringes, which have periods longer than the entire tuning range of the VCSEL. In such a case, the signal variations with ambient temperature resemble a change of the signal offset.
A gas detection device will always contain optical interfaces (at least the laser chip will be sealed hermetically underneath an optical window), and operational conditions will always create a thermal expansion. Therefore, most gas detection devices based on wavelength modulation spectrometry are limited in their lower detection limits by etalon fringes rather than by electronic noise. Etalon fringe suppression is thus a key element in the increase of the performance (accuracy, precision, detection limit) of a gas detection device.
The state of the art knows several techniques for the suppression of etalon fringes.
The first technique consists in the periodical variation of the position of one of the optical components within the gas detection device, preferentially of a mirror. Such a variation can be implemented for example by placing the mirror onto a piezoelectric positioning device and driving said device by an AC voltage (the frequency of the AC voltage being different from the frequency of wavelength modulation). The overall effect of such an implementation is that the etalon fringes undergo a variation by their full amplitude with time. As long as the time constant of the photo detector's amplifier is significantly higher than the period of the piezoelectric AC drive voltage, the amplifier's output signal will average across all possible etalon fringe amplitudes so that changes in the etalon fringes due to thermal expansion do not have any effect. For any given optical setup, the piezoelectric AC drive voltage can be optimized in frequency and in amplitude in order to maximize the etalon fringe suppression.
A second means to suppress etalon fringes consists in modulating the laser wavelength at a second frequency, which has no relation with the original modulation frequency. In a similar way as in the technique described above, the change of the wavelength due to the second modulation causes a temporal variation of the etalon fringe amplitude, which will be averaged by the photodiode's amplifier, provided that amplifier's time constant is significantly higher than the period of the second wavelength modulation. Similarly, the amplitude and frequency of the second wavelength modulation needs to be optimized for maximum etalon fringe suppression for a given optical arrangement.
Other etalon fringe suppression techniques make use of the slow scan in wavelength of the gas absorption peak (while applying the fast wavelength modulation). Here, etalon fringe amplitude and period are numerically calculated from the obtained scan of the 2f signal, which allows them to be cancelled out. The main drawback of this technique is the necessity to acquire a wavelength scan of the 2f amplifier signal across the gas absorption peak, which implies a very slow measurement cycle.
A common point of all etalon fringe suppression techniques is that there is no absolute suppression technique, and that the main performance limitations of most gas detection devices stem from residual etalon fringes. FIG. 2 shows in diagram (a) residual etalon fringes of an oxygen detector (with zero oxygen concentration) which employs a second, independent wavelength modulation (the residual etalon is about 3 times larger than the electronic noise floor) and in diagram (b) the same device as in (a) with three oxygen absorption peaks.
In view of this, it is the object of the present invention to provide an etalon fringe suppression technique for gas detection, which is less dependent from the temperature and sudden wavelength changes.
Based on this etalon suppression technique it is a further object of the invention to provide a method and device with an easier generation of said first measuring signal which is a function of the intensity of said initial light signal and substantially independent of the concentration of said gas.